Colorectal cancer (CRC) is one of the leading causes of cancer death in the United States. Studies indicate that CRC is the consequence of stepwise genetic and epigenetic changes in genes with important functions in regulating proliferation, differentiation, migration and apoptosis of colonic epithelial cells. Examples of changes that lead to CRC include amplification, silencing, and activating or inactivating mutations of protooncogenes or tumor suppressor genes (TSGs), respectively. Thus, somatic or germline inactivation of TSGs such as adenomatous polyposis coli (APC), p53 and several genes involved in DNA mismatch repair; and oncogenic activation of KRAS, BRAF and ?-catenin have been shown to be crucial for the formation of CRC. It is also becoming clear that these changes often result in perturbation of critical signaling pathways such as Wnt, Notch, TGF?, PI3K, and DNA damage response (DDR), which are fundamentally important in the control of colonic epithelial homeostasis. The importance of these pathways is reflected by the manifestation of genetic instability that is present in CRC, of which there are two distinct forms: chromosomal instability (CIN) and microsatellite instability (MIN). Whereas the mechanisms leading to MIN are relatively well delineated, the mechanisms responsible for CIN are still somewhat elusive. Our group previously identified a zinc finger- containing transcription factor called Krppel-like factor 4 (KLF4; also named gut-enriched Krppel-like factor or GKLF), the expression of which is enriched in the terminally differentiated epithelial cells of the small and large intestines. KLF4 is a potent inhibitor of cell proliferation in vitro and does so by eliciting several cell cycle checkpoint responses. This is particularly apparent by the ability of KLF4 to activate both the G1/S and G2/M checkpoints in the cell cycle in a p53-dependent manner following DNA damage. Consistent with its cytostatic effect, KLF4 has been shown to be a potential tumor suppressor of CRC in humans. Additionally, we showed that haploinsufficiency of Klf4 promotes APC-dependent intestinal tumorigenesis in ApcMin/+ mice. Furthermore, we showed that KLF4 is regulated by the Notch signaling pathway which is critical for cell fate determination in the intestine. Lastly, ablation of KLF4 results in genetic instability including centrosome amplification, chromosome aberrations and aneuploidy. Based on these findings, we hypothesize that KLF4 is a critical tumor suppressor of CRC and is involved in maintaining genetic stability. The long-term goal of the proposed project is to understand the role of KLF4 in the pathogenesis of CRC. We propose four specific aims to test our hypothesis: (1) to correlate KLF4 levels with clinical outcome of CRC by tissue microarray analysis (TMA), (2) to establish transgenic mice with intestine-specific ablation of the Klf4 alleles with which to investigate its tumor suppressive function in vivo, (3) to determine the role of KLF4 in regulating the Notch pathway in the intestine, and (4) to determine the mechanisms by which KLF4 maintains genetic stability. The completion of these aims will substantially advance the understanding of the mechanisms of CRC formation.